1. Field of the Invention
The present invention relates generally to RF (radio frequency) transmission. More particularly, the present invention relates to temperature compensating a zero biased diode RF detector circuit.
2. Description of Related Art
Many aspects of radio and transmitting equipment operation rely on the ability to measure relative and absolute RF power levels. It is common practice to measure relative RF power using a detector diode to detect the peak value of RF voltage presented across a known resistance. Absolute RF power can be calculated directly from the diode voltage at a specific temperature. Relative and absolute RF power measurements, however, may be inaccurate due to changes in detector diode characteristics resulting from temperature variation. In particular, at constant current, the forward voltage drop of detector diodes decreases as temperature increases, typically by 2 mV/xc2x0 C. Moreover, the rate of change with respect to temperature is further dependent on the bias current of the detector diodes. Temperature dependent changes, if left uncompensated, result in inaccurate RF power level measurements.
In an attempt to correct this problem, circuits have been developed to compensate for diode characteristic changes due to temperature so that accurate RF power measurements can be made over a wide range of temperatures. A conventional implementation of a temperature compensated diode detector circuit utilizes two identical diodes with the same dc bias current passing through each diode. One diode receives the RF energy (detector diode) to be measured while the other diode does not (reference diode). Temperature related changes in forward bias voltage common to both of the diodes are eliminated from the RF measurements by taking the voltage difference between the two diodes. Taking the voltage difference between the two diodes provides a temperature independent voltage proportional only to the RF power level.
In conventional temperature compensating circuits, external bias must be supplied to the detector and reference diodes in order to provide temperature compensation for RF power measurements. Therefore, there continues to be a need for temperature compensating RF power measurements when dc bias is not supplied or is not available to a RF detection diode.
The temperature compensated zero bias RF detector circuit of the present invention includes a zero biased diode detector circuit feeding the positive input of a differential amplifier circuit. The negative input of the same differential amplifier circuit is a temperature compensation voltage. The temperature compensation voltage is produced by current flow from a bias source through a reference diode and through the resistor back to ground. The bias supply remains constant over temperature, whereas the temperature compensation voltage changes with temperature as the forward voltage across the reference diode changes with temperature. The differential amplifier outputs a temperature compensated detection voltage. The differential amplifier is followed by a voltage level shifter, which adjusts the output temperature compensated voltage to a suitable level for measuring equipment or for use by other processing circuits. Accordingly, the present invention provides temperature compensation of RF power measurements when dc bias is not supplied or is not available to a RF detection diode.